In the words of Walrus Audio:

Unearth the Walrus Audio Silt Harmonic Fuzz, a collaborative creation with tube expert Jim Hagerman. Wielding an internal pre-amp tube, the Silt is a two-in-one fuzz that creates massively rich fuzz tones perfect for chords and rhythmic parts or with the press of a switch, adds uniquely harmonic overtones to the signal before hitting the tube.